A tensioning device, such as a hydraulic tensioner, is used to impart and maintain a certain degree of tension in the chain or belt that drives, for example, an engine camshaft. A hydraulic tensioner typically comprises a housing having a bore, and a piston having a rear end which is inserted into the bore of the housing. The piston is biased in a protruding or outward direction by a spring. A fluid chamber is formed by the inner wall surface of the bore and the rear end face of the piston. A fluid pressure source provides pressurized fluid through a passage in the housing to the fluid chamber.
When the tension of the chain or belt increases during engine operation, the piston receives a force in the retraction direction, and the fluid pressure in the fluid chamber also increases. If the force received by the piston is excessive the piston and housing may be damaged.
In conventional hydraulic tensioners, relief valves have been provided in the housing, and thus require enlargement of the housing. Also, typically, the tip of the piston tends to develop wear from its contact with the tensioner arm.
The present invention is directed to the above-described problems in conventional hydraulic tensioners, and provides a hydraulic tensioner which has a pressure relief valve that releases oil from the fluid chamber to the outside, conserves housing space, and reduces wear of the tip of the piston. When the fluid pressure in the fluid chamber exceeds a predetermined maximum, the pressure relief valve opens so that oil can discharge to the outside and lubricate the tip of the piston.